1. Field of the Invention
The present invention is directed to an X-ray diagnostic installation of the type having an X-ray tube, an X-ray generator, a planar solid state X-ray image converter for generating raw images, an image system and a playback device, wherein the image system includes a device for offset correction that acquires an offset image and stores it in a correction offset memory.
2. Description of the Prior Art
FIG. 1 shows an X-ray diagnostic installation of the above type, as disclosed in German PS 195 27 148, having an X-ray tube 2 supplied with high-voltage and filament voltage by an X-ray generator 1 that emits a conical X-ray beam 3 that penetrates a patient 4 and generates radiation images on a solid state detector 5 that is sensitive to X-rays. The output signal of the solid-state detector 5, the image data 6, is supplied to an image system 7. The image system 7 can include converters, image memories and processing circuits. The image system 7 is connected to a monitor 8 for the playback of the acquired X-ray images. Operating elements 9 are connected to the other components of the X-ray diagnostic installation via a system control and communication 10.
FIG. 2 shows a cross-section of the solid state detector 5 in perspective. The basic components of the solid state detector 5 are composed of a solid state pixel matrix, line drivers and amplifiers. The solid state pixel matrix has a layer with a scintillator 11 composed, for example, of cesium iodide (CsI) that supplies photons in the visible spectrum to a pixel matrix 12 of amorphous silicon when irradiated with the X-ray beam 3, the photons producing a visible X-ray image. As shown enlarged in FIG. 2, each of the pixels or picture elements of this pixel matrix 12 is composed of a photodiode 13 and a switch 14 that is connected to row lines 15 and column lines 16. The pixel matrix 12 is applied on a glass substrate 19.
All pixels of a line are addressed and read out simultaneously by the line drivers 17. The signals are processed in parallel in a number of amplifiers 18. In the simplest case, an image is progressively read out line-by-line.
Newer solid state detectors for X-ray imaging, such as the solid state X-ray image detector 5, are based on active readout matrices of, for example, amorphous silicon (a-Si). The image information is converted in an X-ray converter, for example the scintillator 11, is stored in the photodiodes 13 of the matrix as electrical charge, and is subsequently read out with a dedicated electronics via the active switches 14 and is analog-to-digitally converted. The readout and drive chips, the line drivers 17 and amplifiers 18 are connected to the row lines 15 and column lines 16 of the active pixel matrix 12 with, for example, flexible interconnects (not shown). Electronic noise effects, referred to as microphony effects, can occur due to concussions experienced by the X-ray image converter 5, these being transmitted into the image content due to mechanical oscillations or vibrations during an image acquisition.
Pre-processing steps that are usually undertaken, such as flat fielding, i.e. offset correction and gain correction, as well as fault correction, are shown on the basis of FIG. 3. Since solid state X-ray image converters based on a-Si-like all semiconductor detectorsxe2x80x94exhibit temperature dependencies, current offset images are constantly acquired as background and are stored. This correction offset image stored in memory 20 is normally utilized for correction by means of subtraction in a subtraction stage 22 from the raw image 21 stored in a memory as an unprocessed X-ray image. The offset image is taken into consideration either at 100% or in the form of an averaging with earlier offset images. The result is corrected by multiplication in a multiplication stage 24 with a gain image stored in a memory 23, so that an imaged referred to as a flat-fielded X-ray image is obtained, which is present in a memory 25 for further image processing with further processing electronics 26.
When the currently stored correction offset image in memory 20 (or one of the recent offset images given averaging methods) is disturbed due to the microphony effects caused, for example, by a blow to the exposure stand, pulling out the detector drawer or sitting on the X-ray table, the disturbance is consequently calculated into the further-processing of the X-ray imagexe2x80x94the corrected image exhibits disturbances, artifacts, due to the microphonic effect. These artifacts can be substantial and locally destroy the image content.
Since no such detectors were previously available in the marketplace, the current problem did not arise.
Alternative solutions such as reducing the noise contribution of the current offset image by averaging a number of offset images, or mechanical damping of the detection suspension do not fundamentally solve the problem but only diminish it. Although the first solution would reduce the effect in the individual image, the noise effect would persist over longer times. The disturbance thus would have to be calculated in a number of X-ray images.
German OS 199 15 851 discloses a method wherein an offset-corrected raw image is corrected with reference to artifacts on the basis of data acquired from the dark reference zone in all instances, these artifacts being established by a detector model.
An object of the present invention is to provide an X-ray diagnostic installation of the type initially described wherein the disturbing influence of the microphonic effect is eliminated.
This object is inventively achieved in an installation of the type described above wherein the correction offset memory is preceded by a microphony detector as an analysis module that analyzes the current offset image for microphony disturbances and effects storage only of a current offset image that is disturbance-free. As a result, the current offset image is analyzed for microphony disturbances before a further-processing is permitted. When microphony disturbances are present, the analyzed offset image is discarded and is not utilized for further-processingxe2x80x94the most recently accepted offset image is then still the current one. When it is free of disturbances, i.e. when no microphony effects are detected, a disturbance-free, current offset image is read into the correction offset memory and xe2x80x9cauthorizedxe2x80x9d as a new, current offset image for the correction.
In this way, the problem is solved with relatively little outlay, for example with the assistance of a software analysis, and without complicated and expensive design modifications or improvements in the hardware or mechanics of the detectors or system.
Inventively, the microphony detector can be an analysis device with filters and/or threshold circuits for checking the current offset image for microphony, for example with a high-pass filter and a low-pass filter.
The microphony detector can include a correction device for the current offset image.
Calculating capacity and calculating time can be saved when the microphony detector is fashioned such that it analyzes only a part of the current offset image, for example only in the region of interest (ROI).
It has proven advantageous when the microphony detector is fashioned such that the current offset image is supplied to an offset correction unit, a gain correction unit and/or a fault correction unit. This has the advantage that the resulting image again normally fluctuates around a nominal zero value and that the normal image processing pipeline can be employed.